Family: Poaceae

Origin, Description & Uses
Digitaria insularis, commonly known as sour grass, is a tall, tufted perennial grass native to tropical and subtropical regions of the Americas. It is characterized by narrow, upright leaves and distinctive, feathery seed heads that rise well above the foliage. Sour grass thrives in open, sunny areas and is commonly found along roadsides, in pastures, agricultural fields, and other disturbed sites. It has sometimes been used as forage for livestock and can provide temporary ground cover in degraded soils. However, it is most often regarded as a troublesome agricultural weed rather than a cultivated plant. Sour grass is now widely naturalized on Pacific islands and in Malesia. In the Hawaiian Islands, it is naturalized on Midway, Niʻihau, Kauaʻi, Oʻahu, Molokaʻi, Lānaʻi, Maui, Kahoʻolawe, and Hawaiʻi.

Risks & Threats
Sour grass is considered high risk because it is already naturalized in Hawaiʻi and possesses traits that are cause for concern. It produces abundant wind-dispersed seeds, grows rapidly, and tolerates drought, fire, and poor soils—allowing it to spread aggressively in disturbed and natural areas alike. Dense stands can outcompete native plants, reduce biodiversity, and alter habitat structure. In fire-prone landscapes, sour grass can contribute to increased fuel loads, potentially intensifying wildfire cycles that further favor invasive grasses over native vegetation. Due to its widespread establishment and invasive characteristics, sour grass could continue to detrimentally impact tropical island ecosystems.

High Risk Traits:

• Serious agricultural weed (soybean, rice, pasture)
  
• Naturalized on all main Hawaiian Islands
  
• Weed of disturbed areas (roadsides, fields)
  
• Unpalatable to livestock
  
• Forms dense monocultures
  
• Reproduces by seed and runners
  
• Fast maturation (<1 year)
  
• Wind-dispersed seeds
  
• Persistent seed bank (>1 year)
  
• Herbicide-resistant (glyphosate)
  
• Benefits from fire
  
• Congeneric weeds exist

Low Risk Traits:

• Non-toxic to animals or humans
  
• No spines or thorns
  
• Not intentionally planted